FIELD OF INVENTION
The invention relates to a magnetic holder for an acoustic PD sensor. The invention in particular relates to a device to hold an acoustic sensor during partial discharge measurement on an oil-filled transformer.
BACKGROUND OF THE INVENTION
Partial discharge measurement on an oil-filled transformer is an important test to find out the condition of the oil-paper insulation. The acoustic method of detecting partial discharges in transformers basically employs a sensor. This is less sensitive compared to the electrical method. However, it is quite useful for detecting arcing type discharges in a transformer. A sound electric transducer is provided at one end of the sensor. The sensor also has an in-built pre-amplifier for obtaining higher signal to noise ratio. When the sensor is kept on the transformer tank with the transducer pressing the steel wall of the transformer, it captures the acoustic signal, which is emanating from the PD site and traveling through the oil. The signal is amplified for further analysis. Since the air is a bad conductor of acoustic waves, a gel is introduced between the sensor transducer and the steel tank of the transformer. An optimum pressure need to be applied on the sensor so that the transducer is in perfect contact with the transformer tank and at the same time the gel should not be displaced from the gap. This on-line test is conducted at the sites to locate partial discharges (PD) in power transformers.
The following difficulties are encountered during the PD testing of transformer using this sensor and the associating equipment.
1. Generally, a person who makes the measurement holds a sensor on the steel tank of the transformer some time at a height than the stretched reach of an operator. Because of the application of conducting gel, the sensor slips and it is difficult to hold it without movement. This movement creates sound waves, which is measured along with the PD acoustic waves. Accurate measurement is possible only by experienced persons.

2. Measurement at places near to the top of the transformer needs a
ladder. Also, holding the sensor at this position is tedious.
3. The sensor may fall on the ground and get spoiled.
4. For big transformers at least two experienced persons are required.
OBJECTS OF THE INVENTION
The object of the present invention is to propose a device which can hold a sensor to the steel tank of the transformer thereby make the measurement of partial discharge convenient to the operator.
Another object of the present invention is to propose a device which can be easily fixed and removal at any height on the steel tank of the transformer.
Yet another object of the present invention is to propose a device which is simple, easily operable and protect the sensor from any physical damage.
A further object of the present invention is to propose a device which reduces the total testing and measurement time significantly.
SUMMERY OF THE INVENTION
According to this invention there is provided a magnetic holder for an acoustic PD sensor comprising at least two non-flexible tube members (01,02), the outer member (02) being screwable over the inner member (01), the inner member (01) rigidly holding the acoustic PD sensor (03) ' and having a continuously running groove to accommodate the BNC connector; at least two magnets (04)

housed in a distal end of the inner member (01) configured as a rectangular block (09); a spring means (07) is disposed in the outer member (02) which under tension force ejects the sensor (03) from the rectangular block (09) which when pressably held on the transfer tank, the PD sensor (03) exerting pressure on the transformer tank to capture the acoustic signals traveling through the oil stored in the transformer oil tank for discharge measurement.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. la- shows a schematic view of the preferred embodiment of the present invention in assembled condition.
Fig. lb- An embodiment of Fig. la in pre-assembled condition.
Fig. 2- Prior art device held against the transformer requiring simultaneously canying out the measurement.
Fig.3- A device of the invention where the testing personnel can carry out the measurement without simultaneously holding the transducer against the transformer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention will now be illustrated with reference to Fig. 1(a) and (b). The device comprises two numbers of nylon tubes, one being an inner tube (01) and the other being an outer tube (02). The outer tube (020 being screwable over the inner tube (01) to form a single unit. The inner tube fixedly an acoustic PD sensor (030 having BNC connector. At one end of the inner tube (01), a rectangular groove, running along its length and being opened from same side is provided. The BNC connector of the sensor (03) moves through this groove. The other end of the inner tube (01) is configured as a rectangular block (09) that houses two powerful rectangular magnets (04) which are used for fixing the device on the transformer tank. The insulation thickness between the magnet (040 and the outer surface (02) is between 1.00 to
2.00mm for effective holding of the device against the transformer tank. A rectangular cover (05), which slides over the inner tube (01), covers the magnets (04).
The cover (05) is fixed to the rectangular end(09) of the inner tube (01) by four screws (06) . The outer tube (02) houses a spring, (07) which sits over a circular base (10) and it pushes the sensor (03) out of the inner tube (01) with a pressure. Screwing the outer tube (02) over the inner tube (01) can vary the pressure. The sensor (03) can not come out of the device totally because the end of the rectangular groove stops it. Also, a cover (08) provided over a circular hole (11) in the rectangular block (09) by 2 to 5mm. During the holding position by the magnets (04), the sensor (03) is pushed inside and thereby it exerts a pressure over the steel tank. The other end of the outer tube (02) is culminated into a circular disc (10), which helps as a grip for operator to fix or remove the device.
The holder fixes the sensor at any height on the transformer wall without any movement. This facilitates accurate measurement and also makes measurement easy as shown in Fig. 3. Pressure variation is possible with the action of the spring. Optimum pressure is possible to obtain. The sensor is totally protected from any physical damage. The disc at the back facilities easy handling of the holder. The groove in the inner tube allows BNC cable to connect to the sensor. The cost of the holder is significantly low because of its simple design. The use of this holder at the site reduces the total testing time significantly. Any person can easily operate the equipment. The magnetic holder is presently used for PD measurements on transformers at sites.

WE CLAIM;
1. A magnetic holder for an acoustic PD sensor comprising at least two non-flexible tube members (01,02), the outer member (02) being screwable over the inner member (01), the inner member (01) rigidly holding the acoustic PD sensor (03) and having a continuously running groove to accommodate the BNC connector; at least two magnets (04) housed in a distal end of the inner member (01) configured as a rectangular block (09); a spring means (07) is disposed in the outer member (02) which under tension force ejects the sensor (03) from the rectangular block (09) which when pressably held on the transfer tank, the PD sensor (03) exerting pressure on the transformer tank to capture the acoustic signals traveling through the oil stored in the transformer oil tank for discharge measurement.
2. A magnetic holder for an acoustic PD sensor as claimed in claim 1, wherein the magnets (04) are covered with a rectangular cover (05) sliding over the inner member (01).
3. A magnetic holder for an acoustic PD sensor as claimed in claim 2, wherein the rectangular cover (05) is detachably attached to the inner member (01) by means of a plurality of screws (06).
4. A magnetic holder for an acoustic PD sensor as claimed in claim 1, wherein the rectangular block (09) comprises a circular hole (11) which is provided with a cover (08) isolating and protecting the sensors (04) when not in use.
5. A magnetic holder for an acoustic PD sensor as substantially described herein and illustrated with reference to the accompanying drawings.